With the advance of integrated circuit fabrication technology, ion implantation is achieving increasing acceptance as an expedient for the introduction of the conductivity-determining impurities or dopants into a semiconductor substrate. For many applications, ion implantation has been substituted for the older and more conventional diffusion approach in the introduction of conductivity-determining impurities.
One of the primary advantages of ion implantation in the formation of active and passive regions in the semiconductor substrate is that it offers to the fabricator a higher degree of control of the lateral dimensions and the vertical concentration profiles of the introduced dopants.
The concentration profile is determined by the nature of the ion being implanted, the dosage and the energy level imparted to the ion by the ion implantation equipment. Lateral dimension of the ion implanted region in a substrate are generally determined by the dimensions of apertures in ion implantation barrier masks. Such barrier masks are customarily layers of electrically insulative material having such apertures formed therein. Unlike diffusion steps wherein the lateral borders of the regions of impurities introduced by diffusion extend beyond the lateral limits of the mask of the barrier mask apertures, in ion implantation, the lateral limits of the introduced regions are strictly defined by the aperture in the barrier mask and do not extend beyond the aperture periphery.
We have found that this latter characteristic while it serves to significant advantage in giving strict control of lateral dimensions in high device density integrated circuits, may present a problem where the contours of apertures in ion implantation barrier masks are such that lower portions of a given aperture have greater lateral dimensions than upper portions of the same aperture. In such a case the bottom portion of the aperture will extend laterally beyond the limit of the junction of the region formed by the ion implantation step since lateral limits will be strictly defined by the narrower upper portion of the opening. Such a structure could produce the undesirable effect of a PN junction exposed at the surface of the semiconductor substrate particularly when the electrically insulative layer which serves as the ion implantation barrier mask also serves the usual subsequent purpose of the passivation layer in the final integrated circuit structure.
This problem becomes quite pronounced if an electrical contact to the region formed by ion implantation is to be subsequently made through the deposition of metal into the same opening through which the conductivity-determining impurities were implanted. If there is an exposed PN junction in said opening the metal contact will short across such junction thereby rendering the contact and probably the device ineffective. Even if the metal deposited in the contact does not extend laterally to cover the junction, then the junction will remain exposed and unpassivated and will be subject to severe reduction of transistor gain (Beta).